This invention relates to methods and apparatus for die-casting, and more particularly, to systems for reducing the inclusion of gaseous voids in die castings.
The use of die-casting and plastic molding has been extended to the manufacture of larger and larger articles. Such large automotive parts as internal combustion engine blocks and the housings for automatic transmissions are now commonly manufactured with die-casting as the first step in formation of the part. Such parts have extensive and complex surfaces with close tolerances; and die-casting permits their formation, eliminating costly machining operations and saving metal. Die-casting requires extreme pressures exerted on the liquid metal and large amounts of heat are released from the molten metals as they change state. Massive dies are required to maintain dimensional tolerances within the limits making such operations economically attractive and to provide the strength to withstand the stresses resulting from high pressures and forces. The die-casting molds for such large automotive parts as automatic transmission housings are, for example, frequently seven to eight feet (2.1-2.5 meters) tall, seven to eight feet (2.1-2.5 meters) wide, and six to seven feet (1.8-2.1 meters) thick when closed, and must be manufactured from high-grade, high-tensile strength steel. (The words xe2x80x9cmoldxe2x80x9d and xe2x80x9cdiexe2x80x9d are used interchangeably herein.)
Such molds frequently include one stationary element, one movable element operated by the die-casting machine to close the mold, and several slideable elements referred to as xe2x80x9cslides,xe2x80x9d that move transversely of the direction of movement of the die-casting machine to provide a mold cavity with intricate and re-entrant surface configurations. The mold slides, which slide transversely of the direction of movement of the die-casting machine, are generally moved by hydraulic cylinders to their proper positions.
Die-casting has become desirable as a manufacturing method for parts such as automobile engine blocks and transmission housings because it can produce intricately shaped parts to close tolerances. Thus, die-casting can provide such parts with strength and intricately shaped surfaces without extensive and expensive machining operations. Such parts have wall thicknesses designed to take advantage of the economy of die-casting operations. Misalignment of the mold parts due, for example, to warping of the mold, misalignment of the mold on the molding machine, or non-parallelism in the molding machine platen surfaces or their direction of the movement, can vary wall thicknesses and distort part surface dimensions to unacceptable limits and result in a substantial waste of die-cast parts. In addition, the inclusion of voids within the walls of a casting can create stress concentration sites and can provide undesirably thin areas of the casting""s walls. The detection of voids in casting walls is difficult, and failure to detect poorly cast parts before machining can result in further waste.
In die-casting operations, high pressure is needed to fill quickly the intricate cavities of die-casting molds and to avoid premature solidification of the molten metal as the die cavity is being filled. Die-casting machines typically include a xe2x80x9cshot tubexe2x80x9d connected to a stationary die element so its central cylindrical cavity is in communication with the mold cavity. Molten metal is introduced into the central cavity of the shot tube through a pour hole and is forced into the mold cavity by a piston-driven tip or plunger, (referred to as a xe2x80x9cshot tipxe2x80x9d) that is reciprocally moved in the shot tube cavity. In filling the mold cavity pressures of up to 5500 to 20,000 psi (386-1400 kg/cm2) are exerted by the piston on the molten metal in the shot tube and the mold cavity in each xe2x80x9cshot.xe2x80x9d
In a typical die-casting operation, the shot tube is only partially filled with the volume of metal corresponding to the volume of the die cavity. A shot with a partially filled shot tube is called an xe2x80x9copen shotxe2x80x9d During an open shot, a wave forms in front of the shot tip as it advances. This wave can entrap air bubbles with the molten metal, ultimately resulting in the formation of voids within the casting. Accordingly the shot portion of the die-casting operation, shot tubes and their operations have been the subject of extensive study and development. Examples of such efforts are disclosed in U.S. Pat. No. 5,601,136 and Japanese Patent Publication Nos. 58-148066, 59-921157, 62-101360 and 63-188465, and Chapter 5, Plunger Velocity and Force, of Die Casting Process Engineering and Control, published by The North American Die Casting Association of Rosemont, Ill., 1991. Japanese Patent Publication 63-188465 discloses one attempt to reduce the inclusion of air from the shot tube by adding, to the shot tube, a slot extending from the pour hole in the direction of the die to act as an air vent and reduce the air forced into the die cavity during the shot, but such slots weaken the ability of the shot tube to withstand the high internal pressures exerted on the molten metal and can lead to structural failure of the shot tube and provide an extended avenue for the escape of molten metal as the shot tube tip is advancing, both of which can provide unsafe operating conditions.
To produce higher quality castings, xe2x80x9cclosed shotxe2x80x9d assemblies have been developed. A closed shot tube has a volume corresponding to the volume of the die cavity. Consequently, the sleeve is completely filled with molten metal and the pour hole is closed before the plunger advances. Such closed shot assemblies require complex moving part assemblies that are exposed to the molten metal and extreme pressures and are not preferred in the die-casting industry.
It is believed that none of the prior developments of the shot portion of a die-casting operation and shot tubes and their operation have addressed the combined effects of heat transfer, wave formations, and air within a shot tube during the injection of molten metal into the die cavity. A need continues to exist for a die-casting method and apparatus, which can be operated reliably to substantially reduce or substantially eliminate voids within die cast parts.
The invention provides an improved apparatus and method for injecting molten metal into the die cavity and rests on the belief that air voids within a die casting may be minimized by controlling, during the shot process, the venting of gas from the shot tube and the movement of metal within the shot tube to avoid entrapped gas and prematurely solidified aluminum particles in the molten metal being injected into the die cavity during the shot process.
In the invention a shot tube for use with open shots in injecting molten metal into a die cavity is provided with a vent opening having a diameter DV, preferably from about 18% to about 27% of the diameter of the shot tube, located downstream of the pour opening, and upstream of the distal end of the shot tube a distance LV, which is substantially equal to VM and divided by VT times LT, where VM is the volume of the metal poured into the shot tube, which is somewhat greater than the total volume of the die cavity, and VT and LT are, respectively, the total volume of the shot tube and the length of the shot tube between the shot tip and the distal end of the shot tube (the shot stroke length), and the shot tip is accelerated through the shot tube at a rate maintaining a non-turbulent rising wave of metal in the shot tube until the shot tip is adjacent the vent opening, and is thereafter accelerated to fill the cavity as quickly as possible. Upon filling the cavity, very high pressures are applied to the shot tip by the intensifier to compress the molten metal in the die cavity. In addition, a vacuum can be applied to the die cavity after the shot tip is adjacent the vent opening.
A die casting apparatus of the invention can comprise a die having a part-forming die cavity with a volume VM, a shot tube having a central bore with a length LT and a volume VT connected with a die cavity, a pour opening at the rear of the shot tube for introduction of molten metal into the bore of the shot tube, and a vent opening, preferably having a diameter from about 18% to about 27% of the diameter DT of the shot tube bore, located a distance LV, equal to VM divided by VT times LT, from the distal end of the shot tube connected with the die cavity, the shot tube having a shot tip reciprocatably carried within its bore between its rear and its distal end for urging molten metal into the die cavity, means for introducing at least a volume of VM of molten metal through the pour opening into the shot tube bore between the shot tip and the distal end of the shot tube, and means for advancing the shot tip towards the distal end of the shot tube to force molten metal into the die cavity, said advancing means being operable to accelerate and advance the shot tip at a rate forming a rising but non-turbulent wave of molten metal in front of the shot tip until the shot tip has covered the vent opening and to thereafter advance the shot tip at a rapid rate until the die cavity is filled with molten metal and thereafter being operable to apply extremely high pressure on the molten metal in the die cavity.
The invention also provides a die casting method comprising the steps of providing a shot of molten metal having at least a volume VM, at least equal to the volume of the die cavity, through a pour opening to partially fill the bore of a shot tube having a total volume VT, with the bore of the shot tube being connected with the part-forming cavity of a die, and advancing a shot tip within the bore of the shot tube to form a rising substantially non-turbulent wave of molten material within the shot tube and to expel gas from above the rising wave of molten metal through a vent opening located a distance from the distal end of the shot tube equal to about to VM divided by VT times LT, where VM is a volume of the molten metal of the shot tube, and VT and LT are, respectively, the total volume of the shot tube between its distal end and the shot tip, and the total length of the shot tube between its distal end and the shot tip, and after the shot tip has closed the vent opening rapidly advancing the shot tip to fill the die cavity and intensifying the force applied to the shot tube to exert extreme pressure on the molten metal in the die cavity, preferably at reduced rates. The method can also include applying a vacuum to the die cavity about the time the shot tip has closed the vent opening.